Method of making fine-pitch circuit lines

ABSTRACT

A method of making fine-pitch circuit lines includes steps of preparing an insulative substrate, disposing a conductive metal layer on the insulative substrate, disposing on a whole or a part of a top surface of the conductive metal layer a hetero layer having an etching rate smaller than that of the conductive metal layer, forming a patterned mask of circuit lines on the hetero layer, wet etching the hetero layer and the conductive metal layer, and removing the patterned mask and the hetero layer so as to form fin-pitch circuit lines having a high etching factor on the insulative substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of making a printed circuit board (hereinafter referred to as “PCB”), and more specifically to a method of making a PCB having fine-pitch circuit lines thereon.

2. Description of the Related Art

Conventionally, integrated circuit chips or electronic components are mechanically supported by and electrically connected with a PCB or a wiring substrate for a chip package. For electrical connection with the chips or the electronic components, the circuit board or the substrate is provided at the top thereof with circuit lines forming a specific circuit pattern. It is well known that the circuit pattern is mainly made by wet etching. In the known conventional etching methods, the wet etching, which is developed and adopted by manufacturers long time ago, is nowadays still being used extensively because of its economic advantage. Basically, the production of circuit lines involving a wet etching process includes the steps of disposing a conductive layer on an insulative substrate, forming an etching resist mask having the desired circuit lines on the conductive layer, and removing the areas of the conductive layer that are not protected by the patterned etching resist mask by a strong acid or alkali liquid etchant so as to form the desired circuit lines on the substrate.

Because the liquid etchant used in the wet etching process has an isotropic etching characteristic, the etchant will not only attack the target in a vertical direction but also in a transverse direction, resulting in the so-called undercut phenomenon. Specifically speaking, if the conductive layer is a copper layer and the etchant is FeCl₃ for example, the etchant will also attack the sidewalls of the copper conductive layer that are not protected by the photoresist in addition to the desired vertical etching, causing a mushroom defect.

In practice, the etching quality can be identified by the so-called etching factor. High etching factor represents that the pitch between circuit lines is small, such that fine-pitch or ultra-fine-pitch circuit lines can be realized. FIG. 1 is a schematic drawing illustrating the etching factor. The so-called etching factor is defined as an inverse of a value F, i.e. etching factor is equal to 1/F. While the value F is equal to the equation of (D1−D2)/2H, i.e. F=(D1−D2)/2H; wherein D1 represents the width of the bottom of the circuit line, D2 represents the width of the top of the circuit line and H represents the height of circuit line, i.e. the thickness of the conductive layer. When the etching factor is small, the top of the circuit line is narrow and the bottom is broad. This means that the undercut phenomenon is severe and the pitch between two adjacent circuit lines is reduced, such that an electron migration is likely to occur. In addition, the fine-pitch circuit layout can not be realized due to the sectional area of the circuit line is not in a rectangular shape completely.

To resolve the above-mentioned problems, a solution of forming a granular copper electrodeposit between a copper foil and an insulative substrate is disclosed by Saida et al. in U.S. Pat. No. 5,545,466. According to this patent, the etching factor is enhanced up to about 8.4 to 9.

SUMMARY OF THE INVENTION

As discussed above, it is desired to provide a method that can exactly form fine-pitch circuit lines for a PCB. Therefore, it is an objective of the present invention to provide a method of making fine-pitch circuit lines exhibiting a high etching factor on a PCB substrate or a substrate for a chip package.

Another objective of the present invention is to provide a method of making fine-pitch circuit lines exhibiting a high etching factor even though a conventional etchant is used.

Still another objective of the present invention is to provide a method of making fine-pitch circuit lines, which can reduce the etching time.

To attain the above-mentioned objectives, the method of making fine-pitch circuit lines provided by the present invention comprises the steps of preparing an insulative substrate and then disposing a conductive metal layer on the insulative substrate, disposing on a whole or a part of a top surface of the conductive metal layer a hetero layer having an etching rate smaller than that of the conductive metal layer, forming a patterned mask of circuit lines on the hetero layer, performing wet etching, and removing the patterned mask and the hetero layer so as to form fine-pitch circuit lines having a high etching factor.

Another feature of the method of making fine-pitch circuit lines of the present invention lies in that the conductive metal layer and the hetero layer may be pre-formed into a laminate and the laminate my be disposed on the insulative substrate after the insulative substrate is prepared.

Still another feature of the method of making fine-pitch circuit lines of the present invention lies in that the hetero layer may be disposed on the whole top surface of the conductive metal layer, and then the portion of the hetero layer that is located above a to-be-etched portion of the conductive metal layer is removed after the patterned mask is formed, and the etching process follows thereafter.

Still another feature of the method of making fine-pitch circuit lines of the present invention lies in that the hetero layer may be disposed on the portion of a top surface of the conductive metal layer that is intended not to be etched off after the patterned mask of circuit lines is formed on the top surface of the conductive metal layer, and then the patterned mask is removed and the etching process follows thereafter.

Still another feature of the method of making fine-pitch circuit lines of the present invention lies in that the thickness of the hetero layer is smaller than the thickness of the conductive metal layer. Preferably, the hetero layer may have a thickness of about 0.4 to 1.2 μm under a condition of that the conductive metal layer has a thickness of about 8 μm.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a schematic drawing illustrating the etching factor for a circuit line;

FIG. 2 is a schematic drawing showing a step of the method of making fine-pitch circuit lines according to a first preferred embodiment of the present invention, in which a conductive metal layer and a hetero layer are stacked in succession on an insulative substrate to form a blank PCB;

FIG. 3 is a schematic drawing showing a wet etching step of the method of making fine-pitch circuit lines according to the first preferred embodiment of the present invention, in which a patterned mask is provided on the blank PCB;

FIG. 4 is an electron microscope photo showing a sectional view of a circuit line made by a conventional method of making circuit lines;

FIG. 5 is an electron microscope photo showing a sectional view of a circuit line made by the method of making fine-pitch circuit lines according to the first preferred embodiment of the present invention;

FIG. 6 is a schematic drawing showing a step of the method of making fine-pitch circuit lines according to a second preferred embodiment of the present invention, in which a conductive metal layer is disposed on an insulative substrate and a patterned mask is disposed on the conductive metal layer;

FIG. 7 is a schematic drawing showing another step of the method of making fine-pitch circuit lines according to the second preferred embodiment of the present invention, in which a hetero layer is formed on the portion of the conductive layer that is not covered by the patterned mask; and

FIG. 8 is a schematic drawing showing an etching step of the method of making fine-pitch circuit lines according to the second preferred embodiment of the present invention, in which the patterned mask is removed.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 2-5, the method of making fine-pitch circuit lines according to a first preferred embodiment of the present invention is carried out as follows. First, an insulative substrate 10 made of polyimide is provided. Thereafter, on the insulative substrate 10 a copper conductive layer 20 having a thickness of about 7.97 μm is disposed. And then, a hetero layer 30 having a thickness of about 0.792 μm is disposed on the top surface of the conductive layer 20 by electroplating, chemical vapor deposition or sputtering. It is to be noted that the term “hetero layer” defined in the present invention means that the layer is made of a material different from the material that the conductive layer is made of. In this embodiment, the hetero layer 30 can be made of, but not limited to, nickel (Ni) or tin (Sn). On the other hand, the copper conductive layer 20 and the hetero layer 30 can be pre-formed into a laminate which can be directly disposed on the insulative substrate 10 after the insulative substrate 10 is prepared.

Next, a photoresist (not shown in the drawings) is applied on the top surface of the hetero layer 30 and exposed and developed into a patterned mask 40 having a predetermined circuit line layout. Since the formation of the patterned mask 40 is a well-known prior art, no more detailed description in this regard will be presented hereinafter.

Thereafter, the copper conductive layer 20 and the hetero layer 30 are etched by a liquid etchant of FeCl₃ under a specific temperature, for example in a range of 15 to 45° C. After the circuit lines are formed, the patterned mask 40 is removed and then the hetero layer 30 is removed by an appropriate etchant.

Since the hetero layer 30 made of nickel or tin has an etching rate smaller than that of the copper conductive layer 20 when a conventional FeCl₃ etchant is used in the method of making fine-pitch circuit lines of the present invention, the undercut phenomenon will be minimized because the sidewalls of etched zone of the copper conductive layer 20 will be protected by the hetero layer 30. As a result, the etchant can efficiently attack the target vertically, resulting in that the etching time can be reduced and on the other hand, the difference between the width D2 of top of the circuit line and the width D1 of the bottom of the circuit line can be also reduced, i.e. the etching factor is increased. As shown in FIG. 5, the width D1, the width D2 and the height H of the circuit line, i.e. the thickness of the conductive layer 20, are 8.40 μm, 7.29 μm and 7.97 μm respectively, resulting in that the etching factor is 14.4. On the other hand, the circuit line formed by etching a conventional conductive layer of copper foil has, as shown in FIG. 4, a width D1 of 14.14 μm, a width D2 of 7.41 μm and a conductive layer thickness of 7.09 μm. Therefore, the etching factor, which is calculated from the aforesaid parameters, will be 2.2.

In practice, the method of making fine-pitch circuit lines according to the first embodiment of the present invention can be carried out with a step of removing the portion of the hetero layer 30 that is located above the to-be-etched portion of the conductive layer after the patterned mask is formed on the hetero layer 30, followed by an etching step.

Referring to FIGS. 6-8 again, a method of making fine-pitch circuit lines in accordance with a second preferred embodiment of the present invention is recited hereinafter. First, an insulative substrate 60 is provided and then a copper conductive layer 62 having a predetermined thickness is disposed on the substrate 60. Next, on the top surface of the conductive layer 62 a patterned mask 64 having a specific circuit line layout is formed. And then, a hetero layer 70 is disposed on the portion of the top surface of the conductive layer 62 that is intended not to be etched off, i.e. the portion of the top surface of the conductive layer 62 that is not covered by the patterned mask 64, and then the patterned mask 64 is removed. Finally, an etching step using an etchant of a mixture of hydrogen peroxide and sulfuric acid is carried out and thereafter the hetero layer 62 is removed so as to form fine-pitch circuit lines having a high etching factor.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A method of making fine-pitch circuit lines, comprising the steps of: preparing an insulative substrate; disposing a conductive metal layer on the insulative substrate; disposing on a whole or a part of a top surface of the conductive metal layer a hetero layer having an etching rate smaller than that of the conductive metal layer; forming a patterned mask of circuit lines on the hetero layer; etching the hetero layer and the conductive metal layer with a liquid etchant, and removing the patterned mask and the hetero layer.
 2. The method as claimed in claim 1, wherein the conductive metal layer comprises one selected from the group consisting of copper and a copper alloy.
 3. The method as claimed in claim 2, wherein the hetero layer comprises nickel.
 4. The method as claimed in claim 2, wherein the hetero layer comprises tin.
 5. The method as claimed in claim 1, wherein the hetero layer has a thickness smaller than that of the conductive metal layer.
 6. The method as claimed in claim 3, wherein the hetero layer is disposed on the conductive metal layer by the process selected from the group consisting of electroplating, chemical vapor deposition and sputtering.
 7. The method as claimed in claim 3, wherein the liquid etchant is FeCl₃.
 8. A method of making fine-pitch circuit lines, comprising the steps of: preparing an insulative substrate; preparing a laminate comprising a conductive metal layer and a hetero layer disposed on the conductive metal layer and having an etching rate smaller than that of the conductive metal layer, and disposing the laminate on the insulative substrate in a way that the conductive metal layer is bonded on the insulative substrate; forming a patterned mask of circuit lines on the hetero layer of the laminate; etching the hetero layer and the conductive metal layer with a liquid etchant, and removing the patterned mask and the hetero layer.
 9. The method as claimed in claim 8, wherein the conductive metal layer comprises one selected from the group consisting of copper and a copper alloy.
 10. The method as claimed in claim 9, wherein the hetero layer comprises nickel.
 11. The method as claimed in claim 10, wherein the liquid etchant is FeCl₃.
 12. The method as claimed in claim 9, wherein the hetero layer comprises tin.
 13. The method as claimed in claim 8, wherein the hetero layer has a thickness smaller than that of the conductive metal layer.
 14. A method of making fine-pitch circuit lines, comprising the steps of: preparing an insulative substrate; disposing a conductive metal layer on the insulative substrate; disposing on a top surface of the conductive metal layer a hetero layer having an etching rate smaller than that of the conductive metal layer; forming a patterned mask of circuit lines on the hetero layer; removing the portion of the hetero layer that is located above a to-be-etched portion of the conductive metal layer; etching the to-be-etched portion of the conductive metal layer with a liquid etchant, and removing the patterned mask and the hetero layer.
 15. A method of making fine-pitch circuit lines, comprising the steps of: preparing an insulative substrate; disposing a conductive metal layer on the insulative substrate; forming a patterned mask of circuit lines on the conductive metal layer; disposing on the portion of a top surface of the conductive metal layer that is intended not to be etched off a hetero layer having an etching rate smaller than that of the conductive metal layer; removing the patterned mask; etching the conductive metal layer with a liquid etchant, and removing the hetero layer.
 16. The method as claimed in claim 15, wherein the liquid etchant comprises hydrogen peroxide and sulfuric acid. 